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ECE DepartmentFlorida Institute of Technology
Wireless Data Communication Networks
Lecture 19: Introduction to GSM
ECE DepartmentFlorida Institute of Technology
Outline
Brief history
GSM network architecture
Elements of the air interface organization
Page 2
ECE DepartmentFlorida Institute of Technology
Global System for Mobile Communication Most popular cellular standard
Started as Groupe Special Mobile
2nd generation global digital standard
Important time stamps:
o 1982 CEPT created Groupe Special Mobile
o 1987 MoU – 13 European countries
o 1989 Transferred to ETSI
o 1991 Publishing of Phase 1 specs
o 1991 Radiolinia Finland – first GSM network
o 1993 Telstra Australia – first non-European GSM network
o 1995 US implements GSM in PCS band
o 1996 Russia and China implement GSM
o Since 1996 tremendous growth all over the world
Page 3
Official GSM logo
ECE DepartmentFlorida Institute of Technology
GSM in the World
Page 4
GSM currently holds about 80% of the market share worldwide
ECE DepartmentFlorida Institute of Technology Page 5
GSM Network Layout
GSM architecture is first standardized cellular system architecture
MSCArea
H LR
MSCArea
VLR
MSCTRAUBSC
BTS
BTS
BSS
MSC Area
BSS
BSSBTS
PSTN
PLMN - Public Land Mobile Netw ork
Gatew ayMSC
NSS
GSM network can be divided into three main subsystems: Base Station Subsystem - BSS Mobile Station Subsystem - MSS Network Switching Subsystem - NSS
ECE DepartmentFlorida Institute of Technology Page 6
GSM Network Components
Network has many functional components
Components are integrated through a network protocol – MAP
Majority of GSM interfaces are standardized – allows interworking of equipment from different vendors
Major equipment vendors: Ericsson, Nokia-Siemens, Alcatel-Lucent, Huawei, Motorola, etc.
BSC
BSC
MSC
VLR
HLR
AUC
EIR
MSC VLR
BTS
BTS
BTS
BTS
BTS - Base StationBSC - Base Station ContollerMSC - Mobile Switching CenterVLR - Visitor Location RegisterHLR - Home Location RegisterAUC - Authentification CenterEIR - Equipment Identity Register
Um Interface
A-Bis Interface
AInterface
D
C
E
F
PSTN
B
B,C,D,E,F - MAP Interfaces
ECE DepartmentFlorida Institute of Technology Page 7
Overview of GSM - Mobile Station
Keyboard
Control
Display
Transmit AudioSignal
Processing
Receive AudioSignal
Processing
ChannelDecoding
DeinterleavingM essage
Regenerator
ChannelEncoding
InterleavingM essage
Generator
Ciphering
Ciphering
RFProcessing
RFProcessing
SIM
Duplexer
Antenna
ANTENNAASSEM BLY
TRANSM ITTER
RECEIVER
TRANSCEIVER UNITCONTROLSECTION
Offered as a phone for voice services
Data services will bring new devices to the market
Two functional parts:
HW/SW radio interface
SIM
• Two types of SIM (Subscriber Identity Module)– Smart Card– Plug-in
GSM Mobile Architectural Diagram
ECE DepartmentFlorida Institute of Technology Page 8
Overview of GSM - Base Transceiver Station
BTS is a set of transceivers (TX/RX).
GSM BTS can host up to 16 TX/RX.
In GSM one TX/RX is shared by 8 users.
The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side.
Depending on the application, it can be configured as megacell, macrocell, microcell, pico and femto cell, omni, sectored, etc.
BTS
Typical BTS installations
ECE DepartmentFlorida Institute of Technology Page 9
Overview of GSM - Base Station Controller
Provides a small digital exchange with some mobility tasks
Connects to one or several BTS on the Abis Interface
Connects to the MSC on the A Interface
Designed to offload most of the radio link related processes from the MSC
Provides clock distribution to BTS
Communicates with the OMC
Central Module
SwitchMatrix
Database
OMC
OMC - Operation and MaintenanceCenter
AInterfaceTowards
MSC
AbisInterfaceTowards
BTS
In GSM, BSC handles majority RRM functions• Handoff• Power control• Channel assignment
ECE DepartmentFlorida Institute of Technology Page 10
Overview of GSM - TRAU
TRAU is responsible for trans-coding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec.
It can physically reside on either BSC side or MSC side.
M S
M S
M S
TRAU
BSC MSCTRAU
BSC MSC
BSC MSCTRAU
BTS
BTS
64 kbps
64 kbps
64 kbps
64 kbps
16 kbps16 kbps
16 kbps
16 kbps
BTS
16 kbps
16 kbps
16 kbps
• If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel.
• TRAU, BSC and BTSs form Base Station Subsystem (BSS)
Location of TRAU in GSM
ECE DepartmentFlorida Institute of Technology Page 11
Overview of GSM - MSC
Responsible for connecting the mobile to the landline side
GSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility support
GSM Network can have more than one MSC
• One of the MSC has an added functionality for communication with public network – Gateway MSC (GMSC)
• All calls from the “outside networks” are routed through GMSC
GSM MSC and Gateway MSC
BTS
BSC MSC GMSC
ECE DepartmentFlorida Institute of Technology Page 12
Overview of GSM - HLR/AuC
Database for permanent or semi-permanent data associated with the user
Logically, there is only one HLR per network
Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc.
HLR is usually implemented as an integral part of MSC
AUC is an integral part of HLR responsible for ciphering and encryption.
GSM specifies elaborate encryption schemes.
There are three levels of the encryption: A5/1 – Used by countries in Europe and USA
A5/2 – Used by countries and the so called COCOM list
No encryption – used by all other countries
Introduction to GPRS
ECE DepartmentFlorida Institute of Technology
GSM frequency bands
14 bands defined
Most dominant
o GSM-900 (P-GSM+ E-GSM)
o DCS-1800
o GSM-850 (US)
o PCS-1900 (US)
It is common that a phone is capable of multiband operation
o Dual band - 2 bands
o Quad band - 4 bands
Page 13
ECE DepartmentFlorida Institute of Technology
GSM – Frequency <-> ARFCN calculations
Page 14
ARFCN – Absolute Radio Frequency Channel Number
Example – US PCS allocation
2 2
ECE DepartmentFlorida Institute of Technology
Overview of GSM - Air Interface
GSM is a FDMA/TDMA based technology
Transmissions are discontinuous
Each user is assigned a timeslot
Each frequency is divided into eight timeslots
Each channel has a 200 kHz bandwidth
Overhead signaling is required for coordination and control
Information is sent in bursts
Several types of bursts
Page 15
GSM as a FDMA/TDMA Interface
BTS
USER 1 USER 2 .... USER 8
USER 6 USER 7 USER 8 USER 1
USER 1,ARFCN 1
USER 2,ARFCN 1
USER 8,ARFCN 1
USER 9,ARFCN 2
USER 10,ARFCN 2
USER 16,ARFCN 2
ARFCN 1
ARFCN 2
ECE DepartmentFlorida Institute of Technology Page 16
Overview of GSM - Burst Types
Tail Traffic/Signaling Flag Training Sequence Flag Traffic/Signaling Tail
3 57 1 26 1 57 3
Tail Synchronization Training Sequence Synchronization Tail
3 33939 64
Used to carry information on both control and traffic channels
Mixture of data and overhead
GSM defines 8 training sequences assigned in color code mode
Both on the forward and reverse link
Facilitates the synchronization of the MS to the network at the base band
Commonly referred to as S-burst
Only on the forward link
The same sync sequence is used in all GSM networks
Synchronization Burst
Normal Burst
ECE DepartmentFlorida Institute of Technology Page 17
Overview of GSM - Burst Types
Used when the MS is accessing the system
Shorter in length – burst collision avoidance
Extended synchronization sequence
Used only on the reverse link
• Supports MAHO
• Used to ensure constant power level of the broadcast control channel
• Only on the forward link
Dummy Burst
Access Burst
Tail Predefined Bit Sequence Tail
3 3142
Tail Synchronization Access Bits Tail
8 41 36 3
• GSM mobiles use slotted ALOHA to access the system
• In the case of collision – a hashing algorithm is provided
ECE DepartmentFlorida Institute of Technology Page 18
Overview of GSM - Burst Types
Sometimes referred to as the F-burst
Provides mobile with precise reference to the frequency of the broadcast control channel
Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier
Only on the forward link• Spectral characteristics of the
control channel. • The peak in the spectrum allows
for easier MS network acquisition
Tail Fixed Bit Sequence (All zeros) Tail
3 3142
fc fc+67.7 KHz frequency
Power Spectrum Density
BW = 200KHz
Frequency synchronization burst
ECE DepartmentFlorida Institute of Technology Page 19
Overview of GSM - Physical Channels
Group Name Function Fwd. Link
Rev. Link
BCH BCCH Broadcast control Yes No FCCH Frequency correction Yes No SCH Synchronization Yes No
CCCH PCH Paging Yes No AGCH Access Grant Yes No RACH Random Access No Yes
DCCH SDCCH Standalone dedicated control Yes Yes SAACH Slow-associated control Yes Yes FACCH Fast-associated control Yes Yes
TCH TCH Traffic data and voice Yes Yes
A GSM physical channel can carry several different types of logical channels
Can be divided into two categories: traffic and signaling
Signaling channels can be further categorized as: Broadcast
Common Control
Dedicated Control
ECE DepartmentFlorida Institute of Technology Page 20
Overview of GSM - Frame Hierarchy
Different organization on the superframe level for different logical channels
0 1 2 3 4 5 6 7 21 22 23 24 25
1 TDM A Frame4.615 ms
26 M ultiframe120 ms
51 M ultiframe235.4 ms
51 x 26 Superframe or 26 x 51 Superframe6s 120 ms
Hyperframe3 h 28 min 53 s 760 ms
0 1 2 3 4 48 49 50
0 1 2 3 4 5 6 7 2043 2044 2045 2046 2047
0 1 2 3 4 5 6 7 46 47 48 49 50
0 1 2 3 4 23 24 25
0 1 2 3 4 5 6 7
ECE DepartmentFlorida Institute of Technology Page 21
GSM Time Division Duplex
Communication on the forward and reverse link does not happen simultaneously
Delay of three slots between TX and RX
Time division duplexing avoids RF duplexer at the RF stage
o Reduces the cost of mobile
o Saves battery
0
1 2 3 4 5 6 7 00
1 2 3 4 5765
F o rw a rd L ink - B T S T ra nsm its
R eve rse L in k - M S T ra nsm its
ECE DepartmentFlorida Institute of Technology
Review questions
What are the elements of GSM system?
What is a BTS?
What is a BSC?
What is a MSC?
What is a HLR?
What is a VLR?
What is TRAU?
What is a SIM card?
What is the Abis?
Page 22
What are four most prevalent frequency bands for GSM deployment?
Explain GSM FDMA/TDMA access?
How many time slots are on one GSM ARFCN?
What is an ARFCN?
Name 2 basic channel types in GSM?
Name five burst types used in GSM?
ECE DepartmentFlorida Institute of Technology
Review questions
Draw GSM TDMA hierarchy for traffic channel.
Draw GSM TDMA hierarchy for control channel.
What is the bandwidth of a GSM channel?
What is the difference between DCS-1800 and PCS-1800?
What is a quad-band phone?
Do GSM mobiles use RF duplexers? Please explain.
Page 23